1. Field of the Invention
The present invention relates to an upper substrate and a liquid crystal display apparatus having the upper substrate. More particularly, the present invention relates to an upper substrate with improved reliability and a liquid crystal display apparatus having the upper substrate.
2. Description of the Related Art
Generally, liquid crystal display apparatuses are classified into a transmissive type liquid crystal display apparatus, a reflective type liquid crystal display apparatus and a transmissive and reflective type liquid crystal display apparatus according to light sources used for the apparatuses. The transmissive type liquid crystal display apparatus displays an image using a backlight disposed in a rear face of a liquid crystal cell. The reflective type liquid crystal display apparatus uses natural light as the light source. The transmissive and reflective type liquid crystal display apparatus is operated either by a transmissive display mode or by a reflective display mode. An image is displayed using a light source built in the liquid crystal display apparatus in the transmissive display mode under indoor or dark conditions where no external light source exists. When illumination is sufficient, external light is reflected on a reflective electrode in the liquid crystal display apparatus to display the image in the reflective display mode.
A liquid crystal display apparatus usually controls an arrangement of liquid crystal molecules in accordance with a voltage applied to a liquid crystal layer. The liquid crystal display apparatus is generally classified into a passive matrix type liquid crystal display apparatus and an active matrix type liquid crystal display apparatus according to a method of operation. In the passive matrix type liquid crystal display apparatus, pixels are operated using a root-mean-square value of a voltage difference between a signal line and a scanning line while applying a signal voltage to all the pixels connected to the scanning line. In the active matrix type liquid crystal display apparatus, each pixel is operated by a switching device such as a thin film transistor or a metal-insulator-metal device.
A process for manufacturing an active matrix type and a transmissive type liquid crystal display apparatus using the thin film transistor includes forming a lower substrate having the thin film transistor and a pixel electrode arranged thereon, forming an upper substrate including a color filter and a common electrode, and forming a liquid crystal layer between the lower and upper substrates.
FIG. 1 is a plan view illustrating a conventional transmissive and reflective type liquid crystal display apparatus.
Referring to FIG. 1, the transmissive and reflective type liquid crystal display apparatus operates in a twisted nematic mode in which a first rubbing direction of an upper substrate (or a color filter substrate) 10 is substantially perpendicular to a second rubbing direction of a lower substrate (or a thin film transistor substrate) 20. Conducting a high temperature reliability test at a temperature of about 50 to 60° C. causes inferiority such as a corner white phenomenon. The corner white phenomenon refers to a gray scale changing from medium or black to partially white at a tip of corner of display region DA of the lower substrate 20 along the second rubbing direction.
In a conventional liquid crystal display apparatus, an aperture ratio is increased by superposing a pixel electrode on the data line and on the gate line using an organic insulation layer having a low dielectric constant. The corner white phenomenon may be generated due to impurities caused by the organic insulation layer formed on the lower substrate 20 during rubbing the lower substrate 20. The corner white phenomenon shown at the tip of the lower substrate 20 along the rubbing direction may be prevented by forming a dummy pixel region near the display region DA, thereby shifting a position of a corner white CW toward the black matrix 30.
A conventional transmissive and reflective type liquid crystal display apparatus usually has constant cell gaps in the transmissive mode and in the reflective mode. Since the cell gap is determined based on either the transmissive mode or the reflective mode, a significant difference in image characteristics such as color reproduction may be generated due to an optical path difference between the transmitting light and the reflecting light when the apparatus is used. Thus, a liquid crystal display apparatus having a double cell gap has been developed. A thickness of liquid crystal layer in the transmissive mode is thicker than that of liquid crystal layer in the reflective mode to compensate a transmissivity difference between the reflective mode and the transmissive mode.
Furthermore, technologies of integrating a gate driver circuit and/or a data driver circuit on the lower substrate of the liquid crystal display apparatus have been developed to simplify assembly processes and to reduce size and volume of the liquid crystal display apparatus.
FIG. 2 is a plan view illustrating a liquid crystal display apparatus having a double cell gap with driver circuits integrated thereon. FIG. 3 is a cross-sectional view taken along a line I-I′ of FIG. 2.
Referring to FIGS. 2 and 3, a lower substrate 60 has a display region DA and a driving region DR. Thin film transistors are formed in the display region DA, and a gate driver circuit 68 and/or a data driver circuit (not shown) that operates the thin film transistors is/are formed in the driving region DR. In FIGS. 2 and 3, the thin film transistors, the pixel electrode and other elements formed in the display region of the lower substrate 60 are represented by a reference numeral 64 and positions thereof are omitted.
A spacer (not shown) that separates the upper substrate 50 from the lower substrate 60 by a specific distance to maintain a cell gap is formed on a common electrode 55 corresponding to the display region DA of the upper substrate 50. A capping spacer 90 is formed on the common electrode 55 corresponding to the driving region DR of the upper substrate 50 to improve the reliability of the gate driver circuit 68.
Molecules of liquid crystal is homogeneously arranged in the transmissive and reflective type liquid crystal display apparatus having a double cell gap to set a twist angle of the liquid crystal to be zero. To set the twist angle of the liquid crystal to be zero, the upper substrate 50 is rubbed along a first rubbing direction and the lower substrate 60 is preferably rubbed along a second rubbing direction that is anti-parallel to the first rubbing direction.
A high temperature reliability test conducted at a temperature of about 50 to 60° C. on the transmissive and reflective type liquid crystal display apparatus corresponding to an electrically controlled birefringence mode, in which the first rubbing direction of the upper substrate 50 is anti-parallel with the second rubbing direction of the lower substrate 60, induces a corner white phenomenon CW1 at the tip of corner of the display region DA along the second rubbing direction as well as a corner white phenomenon CW2 at the tip of corner of the upper substrate 250 along the first rubbing direction.
The corner white phenomenon CW2 at the tip of corner of the upper substrate 50 along the first rubbing direction is caused by the capping spacer 90 formed on the upper substrate 50. Impurities ‘B’ caused by rubbing are stacked at side portion of the capping spacer 90 while the capping spacer 90 that improves the reliability of the gate driver circuit 68 formed on the lower substrate 60 makes contact with the upper substrate 50 along the first rubbing direction. The impurities ‘B’ cause the corner white phenomenon CW2 to deteriorate the reliability of the liquid crystal display apparatus.